1. Field of the Invention
The present invention is concerned generally with reworkable high temperature adhesives. More specifically, MCM-D assembly with working temperatures in excess of 350-400xc2x0 C.
2. Description of Related Art
Large area substrate processing is a key solution to improving the productivity of Multi-Clip Module Deposition (MCM-D) technology. The trend of achieving high performance and miniaturizing electronic packaging drives the development of multi-chip module-deposition (MCM-D) technology. However, the high cost of production limits the widespread use of MCM-D technology. One approach to overcoming this limitation is to expand the existing scale (12xc3x9712 in2) of large processing and reuse the costly pallet (carrier substrate) to achieve cost reduction.
Newly developed MCM-D design techniques involve a fully operational process line using a 24xc3x9724 in2 carrier or pallet. During this process, several silicon tiles (8xc3x978 in2) are to be adhered to a large reusable pallet (24xc3x9724 in2), followed by micromechanical fabrication, thin film passivation deposition, and subsequent detachment of the silicon tiles from the pallet. The total weight of each tile-to-pallet assembly is restricted to 10 lbs and the total thickness should be less than 6.25 mm. To achieve this design, a unique high temperature adhesive is needed that remains thermally stable at processing temperature (working temperature in excess of 350-400xc2x0 C.), and then thermally degrades at a slightly higher temperature (reworkable temperature) for silicon tile detachment from the pallet.
Polymeric adhesives are used extensively in present engineering applications due to their attributes of allowing a more uniform stress distribution and a more CTE mismatching material assembly. Among them, epoxy resin and polyimide are two kinds of widely applied adhesives. Polyfunctional epoxies have good adhesion property and can produce cured products having a glass transition temperature (Tg) in excess of 200xc2x0 C. However, even the most highly crosslinked epoxies are unable to tolerate long-term service at temperatures at or above 175xc2x0 C. owing to their intrinsic molecular structure. Although polyimide adhesives can resist high temperature environments in excess of 600xc2x0 C. for short periods, their intractability, solvent volatility, low adhesion strength and water condensation byproduct during cure greatly restrict their application. Therefore, both of these two classic adhesives can not meet the MCM-D requirements.
Lincoln Ying, et al. disclosed thermoplastic polyimide-based reworkable adhesives as die attach for hybrid integrated circuit manufacturing. However, the working temperature window of this type of thermoplastic material is only from room temperature to 275xc2x0 C. Current polymeric high temperature adhesives are predominately polyimide-based that are not reworkable, which places an obstacle to removing the coated substrates from the tile-to-pallet assembly and to reusing the high cost pallets.
The present invention overcomes the above disadvantages by providing reworkable high temperature adhesives, which are not only reworkable at certain required temperatures, but also resist the severe corrosive environment of the MCM-D substrate fabrication and passivation process. The adhesives incorporate a thermally cleavable block in the polymerized chains of the adhesives which allows the thermoset network to be reworkable in a weak acid medium.
The adhesive material is obtained by reacting a thermoplastic adhesive selected from the group consisting of polyetherimides, polyamide-imides, polysulfones, polyethersulfones, silicon-carbon thermosets, polyphenylene sulfides and mixtures thereof, (b) a metal acetonate; (c) an epoxy resin; (d) a crosslinker; and (e) a suitable catalyst.
The adhesives of the present invention are formulated to meet the adhesion, viscosity, thermal stability, and reworkability requirements of MCM-D production.
Accordingly, it is an object of the present invention to provide reworkable adhesives for MCM-D assembly with a working temperature in excess of 350-400xc2x0 C.